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accoustic guitar into tube amp

1/12/2006 6:14 AM
Jack accoustic guitar into tube amp
A friend of mine plays accoustic guitar only and wants my homebrew 15watt(el84) marshall style amp.  
Sounds o.k stock but wondered if anyone had any suggestions how to make it more suitable for accoustic guitars  
1/12/2006 6:49 AM
Carl Gigun
I know a guy who plays an acoustic through his hiwatt sometimes. It sounds okay but the speakers lack the high-end to get that sparkling acoustic sound. I haven't experimented with it but you might try running the amp through a more fullrange speaker, 12" guitar amp speakers hit a brick wall at about 5k.
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1/12/2006 7:30 AM
Andy B.

I heard one speaker that sounded great with accoustics. It had a set of blackback celestions with the wizzer cones in them. The guy said they came from a PA. I would be curious to hear some alnicos like that.  
1/12/2006 10:21 AM
Steve Dallman

With acoustics, you probably want a closed back cabinet, and a tweeter. You need full range, hi-fi type response. An open back speaker cabinet is a midrange device, lacking very low end and having severe dropoff right above the resonant peak, around 5k like Carl G said.  
To be heard in a band, he may need more than least for enough headroom. No matter what the design, a ss rectifier and increased B+ capacitance will help add to the headroom.  
To get a good acoustic tone, you need a good, solid low end, a crisp high end that isn't harsh or strident, and usually the right mid cut.  
If you want to go with only bass and treble, it's hard to beat a Baxandall tone stack.  
If you go with Fender/Marshall type tone controls, I suggest replacing the slope resistor with a 100k audio taper pot in series with a 10k resistor. This "mid slope" control will help dial in the best frequency response.  
If his acoustic doesn't have a preamp, then make the input impedance of the amp higher than 5-10meg. If he has a preamp in the guitar, a stock input is fine.  
Eminence and others make coaxial speakers that allow a treble driver to be screwed onto the back of the speaker.  
Piezos are often used. If you do this, then put a power resistor in series with the + terminal to help prevent burnout. 4-20 ohms at 10 or 20 watts will do.
1/13/2006 8:42 AM
Bob Ingram

I've played an acoustic through my Super Reverb clone and it was a tad dull and lifeless. I performed one small mod that made all the difference in the world. I put a 1uF cap across the tail resistor to add presence, now it just shimmers. Plenty of bottom, a nice smooth top end and not too much on the mids.  
IMO it takes a lot of power to amplify an acoustic right. You need tons of headroom to handle the peaks without getting harsh. The 40 Watt Super is just about right. Mine is from a Weber 6A40 kit.  
1/13/2006 6:27 PM
Carl Gigun

there are a lot of really cheap fullrange speakers out there that are similar to guitar speakers but have response way out to 20k or so. Might be worth trying a couple. Look for ceiling speakers or really cheap 10" fullranges. I don't have time to look around much right now but here's an example. It has lower efficiency than you'd like, but other wise should work:
1/14/2006 8:12 AM
Steve Dallman

There are higher quality ceiling speakers that have a coaxial tweeter instead of just the whizzer cone, but a whizzer cone will give more high end without use of a tweeter.  
Eminence also makes a 10" bass speaker with a whizzer cone specifically so a tweeter wouldn't be necessary in a 4X10 bass cabinet. It isn't bad in efficiency. Somewhere around 95dB/1w@1m.
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